Method and apparatus for effecting gas-liquid contact

ABSTRACT

Components, usually but not exclusively gaseous components, are removed in a liquid medium from gas streams and chemically converted into an insoluble phase or physically removed. Specifically, hydrogen sulfide may be removed from gas streams by oxidation in aqueous chelated transition metal solution in a modified agitated flotation cell. The same principle may be employed with other procedures in which a gaseous phase is dispersed in a liquid phase to effect an interaction between components present in such phases. A gas-liquid contact apparatus, generally a combined chemical reactor and solid product separation device, comprising such modified agitated flotation cell also is described. In order to effect efficient mass transfer and rapid reaction, gas bubbles containing hydrogen sulfide and oxygen are formed by rotating an impeller at a blade tip velocity of at least about 350 in/sec. to achieve the required shear. To assist in the reaction, a surrounding shroud has a plurality of openings, generally of aspect ratio of approximately 1, of equal diameter and arranged in uniform pattern, such as to provide a gas flow therethrough less than about 0.02 lb/min/opening in the shroud. In general, the gas velocity index of gas through the openings in the shroud is at least about 18 per second per opening, preferably at least about 24 per second per opening. Each of the openings has an area corresponding to an equivalent diameter less than about one inch.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 622,485 filed Dec. 5, 1990 (now U.S. Pat. No. 5,174,973) whichitself is a continuation-in-part of U.S. patent application Ser. No.582,423 filed Sep. 14, 1990 (now abandoned) which itself is acontinuation-in-part of U.S. patent application Ser. No. 446,776 filedDec. 6, 1989 (now abandoned).

FIELD OF INVENTION

The present invention relates to method and apparatus for effectinggas-liquid contacting, for example, for the removal of components fromgas streams, in particular by chemical conversion of gaseous componentsto an insoluble phase while in contact with a liquid phase or slurry, orfor the removal of components from a liquid phase.

BACKGROUND TO THE INVENTION

Procedures for effecting contact between a gas phase and a liquid phasehave been devised for a variety of purposes, for example, to reserve acomponent from the gas or liquid phase. In this regard, many gas streamscontain components which are undesirable and which need to be removedfrom the gas stream prior to its discharge to the atmosphere or furtherprocessing. One such gaseous component is hydrogen sulfide, whileanother such component is sulfur dioxide.

Hydrogen sulfide occurs in varying quantities in many gas streams, forexample, in sour natural gas streams and in tail gas streams fromvarious industrial operations. Hydrogen sulfide is odiferous, highlytoxic and a catalyst poison for many reactions and hence it is desirableand often necessary to remove hydrogen sulfide from such gas streams.

There exist several commercial processes for effecting hydrogen sulfideremoval. These include processes, such as absorption in solvents, inwhich the hydrogen sulfide first is removed as such and then convertedinto elemental sulfur in a second distinct step, such as in a Clausplant. Such commercial processes also include liquid phase oxidationprocesses, such as Stretford, LO-CAT, Unisulf, Sulferox, Hiperion andothers, whereby the hydrogen sulfide removal and conversion to elementalsulfur normally are effected in reaction and regeneration steps.

In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No.4,919,914, the disclosure of which is incorporated herein by reference,there is described a process for the removal of hydrogen sulfide fromgas streams by oxidation of the hydrogen sulfide at a submerged locationin an agitated flotation cell in intimate contact with an iron chelatesolution and flotation of sulfur particles produced in the oxidationfrom the iron chelate solution by hydrogen sulfide-depleted gas bubbles.

The combustion of sulfur-containing carbonaceous fuels, such as fueloil, fuel gas, petroleum coke and coal, as well as other processes,produces an effluent gas stream containing sulfur dioxide. The dischargeof such sulfur dioxide-containing gas streams to the atmosphere has leadto the incidence of the phenomenon of "acid rain", which is harmful to avariety of vegetation and other life forms. Various proposals have beenmade to decrease such emissions.

A search in the facilities of the United States Patent and TrademarkOffice with respect to gas-liquid contacting procedures has revealed thefollowing U.S. patents as the most relevant to the present invention:

    ______________________________________                                        U.S. Pat. No. 2,274,658                                                                       U.S. Pat. No. 2,294,827                                       U.S. Pat. No. 3,273,865                                                                       U.S. Pat. No. 4,683,062                                       U.S. Pat. No. 4,789,469                                                       ______________________________________                                    

U.S. Pat. Nos. 2,274,658 and 2,294,827 (Booth) describe the use of animpeller to draw gas into a liquid medium and to disperse the gas asbubbles in the liquid medium for the purpose of removing dissolvedgaseous materials and suspended impurities from the liquid medium,particularly a waste stream from rayon spinning, by the agitation andaeration caused by distribution of the gas bubbles by the impeller.

The suspended solids are removed from the liquid phase by frothflotation while the dissolved gases are stripped out of the liquidphase. The process described in this prior art is concerned withcontacting liquid media in a vessel for the purpose of removingcomponents from the liquid phase by the physical actions of strippingand flotation.

These references contain no discussion or suggestion for removal ofcomponents from gas streams by introduction to a liquid phase or thetreatment of components dissolved or suspended in the liquid phase bychemical interaction with components of the gas phase. In addition, thereferences do not describe any critical combination of impeller--shroudparameters for effecting such removal, as required herein.

U.S. Pat. No. 3,273,865 describes an aerator for sewage treatment. Ahigh speed impeller in the form of a stack of flat discs forms a vortexin the liquid to draw air into the aqueous phase and circulate theaqueous phase. As in the case of the two Booth references, this priorart does not describe or suggest an impeller-shroud combination foreffecting the removal of components from a liquid phase or gaseousphase, as required herein.

U.S. Pat. No. 4,683,062 describes a perforated rotatable body structurewhich enables liquid/solid contact to occur to effect biocatalyticalreactions. This reference does not describe an arrangement in whichgas-liquid contact is effected.

U.S. Pat. No. 4,789,469 describes the employment of a series of rotatingplates to introduce gases to or remove gases from liquids. There is nodescription or suggestion of an impeller-shroud combination, as requiredherein.

Many other gas-liquid contactors and flotation devices are described inthe literature, for example:

(a) "Development of Self-Inducing Dispenser for Gas/Liquid andLiquid/Liquid Systems" by Koen et al, Proceeding of the Second EuropeanConference on Mixing, Mar. 30, 1977-Apr. 1, 1977;

(b) Chapter entitled "Outokumpu Flotation Machines" by K. Fallenius, inChapter 29 of "Flotation", ed. M. C. Fuerstenau, AIMM, PE Inc, New York1976; and

(c) Chapter entitled "Flotation Machines and Equipment" in "FlotationAgents and Processes, Chemical Technology Review #172", M. M. Ranney,Editor, 1980.

However, none of this prior art describes the impeller-shroud structureused herein.

SUMMARY OF THE INVENTION

In the present invention, a novel procedure is provided for effectinggas-liquid contact between a gas and a liquid which employs a rotatoryimpeller and shroud combination operated under specific conditions toeffect rapid mass transfer between gaseous and liquid phases and therebyachieve an enhanced efficiency of removal of a component from the gas orliquid phase or transfer of a component one to the other by chemicalreaction, absorption or desorption.

In one aspect of the present invention, there is provided a method forthe distribution of a gaseous phase in a liquid phase using a rotaryimpeller comprising a plurality of blades at a submerged location in theliquid phase surrounded by a shroud through which are formed a pluralityof openings.

The impeller is rotated about a substantially vertical axis at thesubmerged location within the liquid phase at a blade tip velocity of atleast about 350 in/sec, preferably about 500 to about 700 in/sec, anddraws liquid phase to the interior of the shroud.

A gaseous phase is fed to the submerged location and the shear forcesbetween the impeller blades and the plurality of openings in the shrouddistributes the gaseous phase in the liquid phase as fine bubbles to theinterior of the shroud and to form a gas-liquid mixture of fine bubblesof the gaseous phase in liquid phase contained within the shroud and toeffect intimate contact of gas and liquid phases at the submergedlocation and initiate rapid mass transfer.

The gas-liquid mixture of fine bubbles of gaseous phase and liquid phaseflows from the interior of the shroud through and in contact with theopenings therein to external of the shroud at a gas velocity index (GVI)of at least about 18 per second per opening, preferably at least about24 per second per opening which causes further shearing of the fine gasbubbles and further intimate contact of gaseous phase and liquid phase.

The gas velocity index (GVI) is the ratio of the linear velocity (V) ofthe gaseous phase through each opening and the equivalent diameter (d)of the opening, as determined by the expression:

    GVI=V/d

where d is determined for each opening by the expression:

    d=4A/P

where A is the area of the opening and P is the length of the perimeterof the opening.

By employing the unique combination of impeller blade tip velocity andgas flow index through the shroud openings as set forth herein, a veryefficient distribution of gas and liquid phases is effected, such thatrapid and efficient mass transfer occurs. As noted above and asdescribed in detail below, this result may be employed in a variety ofapplications where such rapid and efficient mass transfer is desirableand can be effected in the region of the shroud, as opposed to the bodyof the liquid medium, as in the case of mineral separation.

Such procedures include:

(a) the removal of gaseous components from gas streams, in particular bychemical conversion of such gaseous components or by physicaldissolution of such gaseous components,

(b) the removal of dissolved components from a liquid phase, inparticular by chemical conversion of the dissolved components by gaseouscomponents of the gas stream or physical desorption of dissolvedcomponents, and

(c) the treatment of suspended components in the liquid phase, inparticular by chemical treatment with gaseous components of the gasstream.

The enhanced efficiency which is achieved in the present inventionresults from dispersion of fine bubbles of gas phase in the liquidphase, formation of an intimate mixture of gaseous and liquid phasesconfined within the shroud and passage of the intimate mixture throughand in contact with the shroud, such as to achieve rapid mass transferof interactive components one to the other.

Also as discussed in more detail below, the equipment used in the methodof the present invention, has a superficial similarity to flotationequipment generally employed for the separation of suspended solidcomponents from a liquid phase. However, the present invention employsequipment modifications and operating parameter modifications notemployed in such flotation operations.

The present invention, in another aspect, provides a novel gas-liquidcontact apparatus which is useful for effecting the method broadlydescribed above and in more detail below. Such apparatus comprises tankmeans, inlet gas manifold means for feeding at least one gas streamthrough an inlet to the tank means for holding a liquid phase andstandpipe means communicating with the inlet and extending downwardlywithin the tank to permit a gas to be fed to a submerged location in theliquid phase.

Impeller means comprising a plurality of blades is located towards thelower end of said standpipe means and hence at the submerged locationand is mounted to a shaft for rotation about a generally vertical axisby drive means. Such drive means may comprise an external drive motor oran in-line impeller driven by the pressure of the gas stream fed to theapparatus. Shroud means surrounds the impeller means and has a pluralityof openings extending through the wall of the shroud means.

Each of the openings through the shroud means has an equivalent diameter(d) such that the ratio of the equivalent diameter to the diameter ofthe impeller means is less than about 0.15. The equivalent diameter foreach opening is determined by the expression:

    d=4A/P

where A is the area of the opening and P is the length of the perimeterof the opening.

Preferably, the equivalent diameter (d) for each opening is less thanabout 1 inch and the openings are arranged to permit the flow of gas andliquid phase through the shroud openings at a gas velocity index (GVI)of at least about 18 per second per opening in the shroud.

The shroud construction comprises an additional aspect of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an upright sectional view of a novel gas-liquid contactapparatus provided in accordance with one embodiment of the invention;

FIG. 2 is a detailed perspective view of the impeller and shroud of theapparatus of FIG. 1;

FIG. 3 is a close-up perspective view of a portion of the shroud of FIG.2; and

FIG. 4 is a upright sectional view of a novel gas-liquid contactapparatus provided in accordance with a second embodiment of theinvention.

GENERAL DESCRIPTION OF INVENTION

The present invention is directed, in one embodiment, towards improvingthe process of the prior Canadian Patent No. 1,212,819 by modificationto the physical structure of the agitated flotation cell employedtherein and of the operating conditions employed therein, so as toimprove the overall efficiency of hydrogen sulfide removal and therebydecrease operating and capital costs, while, at the same time, retaininga high efficiency for removal of hydrogen sulfide from the gas stream.

However, the present invention is not restricted to effecting theremoval of hydrogen sulfide from gas streams by oxidation, but ratherthe present invention is generally applicable to the removal of gas,liquid and/or solid components from a gas stream by chemical reaction,and more broadly includes the removal of gaseous phase components in anyphysical form as well as sensible heat from a gas stream by gas-liquidcontact.

In one embodiment of the present invention, a gas stream is brought intocontact with a liquid phase in such a manner that there is efficientcontact of the gas stream with the liquid phase for the purpose ofremoving components from the gas stream, particularly an efficientcontact of gas and liquid is carried out for the purpose of effecting areaction which removes a component of the gas and converts thatcomponent to an insoluble phase while in contact with the liquid phase.However, the removal of a component may be effected by a physicalseparation technique, rather than a chemical reaction. These operationscontrast markedly with the conventional objective of the design of aflotation cell, which is to separate a slurry or suspension into aconcentrate and a gangue or barren stream in minerals beneficiation. Acomponent is not specifically removed from a gas stream during thelatter operations, nor is there an interaction of gaseous phase andliquid phase components. The distribution of the gas phase in the liquidphase in such flotation processes is for the sole purpose of physicalremoval of the gas solid phase by flotation by gaseous bubbles.

There are a variety of processes to which the principles of the presentinvention can be applied. The processes may involve reaction of agaseous component of the gas stream with another gaseous species in aliquid phase, usually an aqueous phase, often an aqueous catalystsystem.

One example of such a process is the oxidative removal of hydrogensulfide from gas streams in contact with an aqueous transition metalchelate system to form sulfur particles, as described generally in theabove-mentioned Canadian Patent No. 1,212,819.

Another example of such a process is in the oxidative removal ofmercaptans from gas streams in contact with a suitable chemical reactionsystem to form immiscible liquid disulfides.

A further example of such a process is the oxidative removal of hydrogensulfide from gas streams using chlorine in contact with an aqueoussodium hydroxide solution, to form sodium sulphate, which, after firstsaturating the solution, precipitates from the aqueous phase.

An additional example of such a process is the removal of sulfur dioxidefrom gas streams by the so-called "Wackenroder's" reaction by contactinghydrogen sulfide with an aqueous phase in which the sulfur dioxide isinitially absorbed, to form sulfur particles. This process is describedin U.S. Pat. Nos. 3,911,093 and 4,442,083. The procedure of the presentinvention also may be employed to effect the removal of sulfur dioxidefrom a gas stream into an absorbing medium in an additional gas-liquidcontact vessel.

A further example of such a process is the removal of sulfur dioxidefrom gas streams by reaction with an aqueous alkaline material.

The term "insoluble phase" as used herein, therefore, encompasses asolid insoluble phase, an immiscible liquid phase and a component whichbecomes insoluble when reaching its solubility limit in the liquidmedium after start up.

The component removed from the gas stream in this embodiment of theinvention usually is a gaseous component but the present inventionincludes the removal of other components from the gas stream, such asparticulate material or dispersed liquid droplets.

For example, the present invention may be employed to remove solidparticles or liquid droplets from a gas stream, i.e. aerosol droplets,such as by scrubbing with a suitable liquid medium. Similarly, moisturemay be removed from a gas stream, such as by scrubbing with a suitablehydrophilic organic liquid, such as glycol.

A wide range of particle sizes from near molecular size through Aikinnuclei to visible may be removed from a gas stream by the wellunderstood mechanisms of diffusion, interception, impaction and capturein a foam layer using the method described herein.

More than one component of any type and components of two or more typesmay be removed simultaneously or sequentially from the gas stream. Inaddition, a single component may be removed in two or more sequentialoperations.

The present invention also may be employed to remove sensible heat (orthermal energy) from a gas stream by contacting the gas stream with asuitable liquid phase of lower temperature to effect heat exchange.Similarly, sensible heat may be removed by evaporation of a liquidphase.

Accordingly, in one preferred aspect of the present invention, there isprovided a method of removing a component from a gas stream containingthe same in a liquid phase, comprising a plurality of steps. Acomponent-containing gas stream is fed to an enclosed gas-liquid contactzone in which is located a liquid medium.

An impeller comprising a plurality of blades is rotated about agenerally vertical axis at a submerged location in the liquid medium soas to induce flow of the gas stream along a generally vertical flow pathfrom external to the gas-liquid contact zone to the submerged location.

The impeller is surrounded by a shroud through which are formed aplurality of openings, generally within a preferred range of impeller toshroud diameter ratios found in flotation cells. The impeller is rotatedat a speed corresponding to a blade tip velocity of at least about 350in/sec., preferably about 500 to about 700 in/sec., so as to generatesufficient shear forces between the impeller blades and the plurality ofopenings in the shroud to distribute the gas stream as fine gas bubblesin the liquid medium to the interior of the shroud, thereby achievingintimate contact of the component and liquid medium at the submergedlocation so as to form a gas-liquid mixture of fine gas bubbles in saidliquid medium contained within the shroud and to effect removal of thecomponent from the gas stream into the liquid medium.

The gas-liquid mixture of fine gas bubbles and liquid medium flows fromthe interior of the shroud through and in contact with the openingstherein into the body of the liquid medium external to the shroud at agas velocity index (GVI) at approximately atmospheric pressure of atleast about 18 per second per opening, preferably at least about 24 persecond per opening, so as to effect further shearing of the fine gasbubbles and further intimate contact of the gas stream and the liquidmedium, whereby any removal of component not effected in the interior ofthe shroud is completed in the region of the liquid medium adjacent tothe exterior of the shroud.

In this embodiment, as well as the other embodiments of the invention,the gas velocity index (GVI) more preferably is at least about 30 persecond per opening, and may range to very high values, such as up toabout 400 per second per opening, and often is in excess of about 100per second per opening.

As mentioned above, the gas velocity index (GVI) per opening is theratio of linear velocity of the gas phase through each opening and theequivalent diameter of the opening, as determined by the relationship:##EQU1## where the equivalent diameter (d) is determined by therelationship: ##EQU2##

A component-depleted gas stream is vented from a gas atmosphere abovethe liquid level in the gas-liquid contact zone to exterior of theenclosed gas-liquid contact zone.

While the gas-liquid contact procedure is generally operated with anenclosed reaction zone operating at or near atmospheric pressure, italso is possible to carry out the gas stream component removal methodunder superatmospheric and subatmospheric conditions, depending oncircumstances and requirements.

While the present invention, in the gas stream component removalembodiment, is described specifically with respect to the removal ofhydrogen sulfide and sulfur dioxide from gas streams containing the sameby reaction to form sulfur and recovery of the so-formed sulfur byflotation by bubbles of the component-depleted gas stream, it will beapparent from the foregoing and subsequent discussion that both theapparatus provided in accordance with an aspect of the present inventionand the gas stream component removal method embodiment of the inventionare useful for effecting other procedures where a component of a gasstream is removed in a liquid medium or a component of a liquid mediumis removed from by reaction with a gaseous component. In addition, itwill be apparent that the present invention broadly relates to methodand apparatus for distribution of a gas phase in a liquid phase for avariety of purposes.

In one preferred aspect of the invention, hydrogen sulfide contained ina gas stream is converted to solid sulfur particles by oxygen in anaqueous transition metal chelate solution as a reaction medium. Theoxygen employed in this conversion process is present in anoxygen-containing gas stream which is introduced to the same submergedlocation in the aqueous catalyst solution as the hydrogensulfide-containing gas stream, either in admixture therewith or as aseparate gas stream. The oxygen-containing gas stream similarly isdistributed as fine bubbles by the rotating impeller, which achievesintimate contact of oxygen and hydrogen sulfide with each other and theaqueous catalyst solution to effect the oxidation. The hydrogen sulfide,therefore, is removed by chemical conversion to insoluble sulfurparticles.

The solid sulfur particles are permitted to grow or are subjected tospherical agglomeration or flocculation until they are of a size whichenables them to be floated from the body of the reaction medium to thesurface thereof by hydrogen sulfide-depleted gas bubbles.

The sulfur is of crystalline form and particles of sulfur aretransported by the hydrogen-sulfide depleted gas bubbles from thereaction medium to the surface thereof when having a particle size offrom about 10 to about 50 microns in diameter to form a sulfur frothfloating on the surface of the aqueous medium and a hydrogensulfide-depleted gas atmosphere above the froth, from which is vented ahydrogen sulfide-depleted gas stream. The sulfur-bearing froth isremoved from the surface of the aqueous medium to exterior of theenclosed reaction zone, either on a continuous or intermittent basis.

Sulfur formed in such hydrogen sulfide-removing processes and providedas a froth on the surface of the liquid phase has been found to behighly adsorbent of other odiferous components, such as odiferoussulfurous and/or nitrogenous compounds, not oxidized and hence removedduring the hydrogen sulfide oxidation. This result makes the processparticularly useful in the treatment of exhaust gas streams from meatrendering plants, which contain a large variety of odiferous sulfur andnitrogen compounds, in addition to hydrogen sulfide, which are adsorbedby the sulfur on the surface of the reaction medium and hence areremoved from the gas stream, thereby permitting an odour-reduced gasstream to be vented from the plant. The use of freshly precipitated highsurface area sulfur for the removal of odiferous gases from gas streamsis an additional aspect of the present invention.

Accordingly, in an additional aspect of the present invention, there isprovided a continuous method for the removal of components from a gasstream comprising a component oxidizable to sulfur in an aqueouscatalyst-containing medium and odiferous components not oxidizable insaid aqueous medium, which comprises continuously forming sulfur in anaqueous phase from said component oxidizable to sulfur and collectingsaid continuous-formed sulfur as a froth on the surface of said gaseousphase, continuously passing said gas stream from said aqueous phasethrough said sulfur froth to contact sulfur in said froth and adsorbodiferous components from said gas stream, and continuously removingsulfur froth from the surface of said aqueous phase.

Since sulfur is formed continuously from the hydrogen sulfide or othersulfur-forming component and floated from the liquid phase, the sulfurin the froth on the surface of the liquid is continuously recovered, sothat the odiferous compounds continuously contact fresh sulfur in thefroth.

High levels of hydrogen sulfide removal efficiency are attained usingthe method of the present invention, generally in excess of 99.99%, fromgas streams containing any concentration of hydrogen sulfide. Residualconcentrations of hydrogen sulfide less than 0.1 ppm by volume can beattained. Corresponding removal efficiencies are achieved for theremoval of other gaseous components from gas streams.

The method of the invention is able to remove effectively hydrogensulfide from a variety of different source gas streams containing thesame, provided there is sufficient oxygen present and dispersed in thereaction medium to oxidize the hydrogen sulfide. The oxygen may bepresent in the hydrogen sulfide-containing gas stream to be treated ormay be separately fed, as is desirable where natural gas or othercombustible gas streams are treated.

Hydrogen sulfide-containing gas streams which may be processed inaccordance with the invention include fuel gas and natural gas and otherhydrogen sulfide-containing streams, such as those formed in oilprocessing, oil refineries, mineral wool plants, kraft pulp mills, rayonmanufacturing, heavy oil and tar sands processing, coking coalprocessing, meat rendering, a foul gas stream produced in themanufacture of carborundum and gas streams formed by air strippinghydrogen sulfide from aqueous phases. The gas stream may be onecontaining solids particulates or may be one from which particulates areabsent. The ability to handle a particulate-laden gas stream in thepresent invention without plugging may be beneficial, since thenecessity for upstream cleaning of the gas is obviated.

The method of the present invention as it is applied to effectingremoval of hydrogen sulfide from a gas stream containing the samegenerally employs a transition metal chelate in aqueous medium as thecatalyst for the oxidation of hydrogen sulfide to sulfur. The transitionmetal usually is iron, although other transition metals, such asvanadium, chromium, manganese, nickel and cobalt may be employed. Anydesired chelating agent may be used but generally, the chelating agentis ethylenediaminetetraacetic acid (EDTA). An alternative chelatingagent is HEDTA. The transition metal chelate catalyst may be employed inhydrogen or salt form. The operative range of pH for the processgenerally is about 7 to about 11.

The hydrogen sulfide removal process of the invention is convenientlycarried out at ambient temperatures of about 20° to 25° C., althoughhigher and lower temperatures may be adopted and still achieve efficientoperation. The temperature generally ranges from about 5° to about 80°C.

The minimum catalyst concentration to hydrogen sulfide concentrationratio for a given gas throughput may be determined from the rates of thevarious reactions occurring in the process and is influenced by thetemperature and the degree of agitation or turbulence in the reactionvessel. This minimum value may be determined for a given set ofoperating conditions by decreasing the catalyst concentration until theremoval efficiency with respect to hydrogen sulfide begins to dropsharply. Any concentration of catalyst above this minimum may be used,up to the catalyst loading limit of the system.

The removal of hydrogen sulfide by the process of the present inventionis carried out in an enclosed gas-liquid contact zone in which islocated an aqueous medium containing transition metal chelate catalyst.A hydrogen sulfide-containing gas stream and an oxygen-containing gasstream, which usually is air but may be pure oxygen or oxygen-enrichedair, are caused to flow, either separately or as a mixture, along avertical flow path from outside the gas-liquid contact zone to asubmerged location in the aqueous catalyst medium, from which themixture is forced by the rotating impeller to flow through the shroudopenings into the body of the aqueous medium. The rotating impeller alsodraws the liquid phase from the body of aqueous medium in the enclosedzone to the location of introduction of the gas streams, interior of theshroud.

As described above, the gas streams are distributed as fine bubbles bythe combined action of the rotating impeller and the surrounding shroudwhich has a plurality of openings therethrough. To achieve goodgas-liquid contact and hence efficient oxidation of hydrogen sulfide tosulfur, the impeller is rotated rapidly so as to achieve a blade tipvelocity of at least about 350 in/sec, preferably about 500 to about 700in/sec. In addition, shear forces between the impeller and thestationary shroud assist in achieving the good gas-liquid contact byproviding a gas velocity index (as defined above) which is at leastabout 18 per second per opening, preferably at least about 24 per secondper opening. In this aspect of the invention and the others describedherein, other than at or near the upper limit of capacity of a unit, thegas flow rate through the openings is less than about 0.02lb/min/opening in the shroud, generally down to about 0.004, andpreferably in the range of about 0.005 to about 0.007 lb/min/opening inthe shroud.

The distribution of the gases as fine bubbles in the reaction medium inthe region of the impeller enables a high rate of mass transfer tooccur. In the catalyst solution, a complicated series of chemicalreactions occurs resulting in an overall reaction which is representedby the equation:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

The overall reaction thus is oxidation of hydrogen sulfide to sulfur.

As noted earlier, the solid sulfur particles grow in size until of asize which can be floated. Alternative procedures of increasing theparticle size may be employed, including spherical agglomeration orflocculation. The flotable sulfur particles are floated by the hydrogensulfide-depleted gas bubbles rising through the body of catalystsolution and collected as a froth on the surface of the aqueous medium.The sulfur particles range in size from about 10 to about 50 microns indiameter and are in crystalline form.

The series of reactions which is considered to occur in the metalchelate solution to achieve the overall reaction noted above is asfollows:

    H.sub.2 S=H.sup.+ +HS.sup.-

    OH.sup.- +FeEDTA.sup.- =[Fe.OH.EDTA].sup.=

    HS.sup.- +[Fe.OH.EDTA.sup.- ]=[Fe.HS.EDTA].sup.= +OH.sup.-

    [Fe.HS.EDTA].sup.= =FeEDTA.sup.- +S +H.sup.+ +2e

    2e+1/2O.sub.2 +H.sub.2 O=2OH.sup.-

Alternatively, the oxygen-containing gas stream may be introduced to themetal chelate solution at a different submerged location from thehydrogen sulfide-containing air stream using a second impeller/shroudcombination, as described in more detail in copending U.S. patentapplication Ser. No. 709,158 filed Jun. 3, 1991 ("Dual Impeller"),assigned to the assignee hereof, the disclosure of which is incorporatedherein by reference.

In another preferred aspect of the present invention, sulfur dioxide isreacted with an alkaline medium to remove the sulfur dioxide from a gasstream bearing the same. Sulfur dioxide is absorbed from the gas streaminto the aqueous alkaline medium and reacts with active alkali thereinto form salts, with the sulfur dioxide-depleted gas stream being ventedfrom the reaction medium. The procedure shows many similarities with thehydrogen sulfide-removal procedure just described, except that theaqueous medium contains an alkaline material.

The aqueous alkaline medium into which the sulfur dioxide-containing gasstream is introduced may be provided by any convenient alkaline materialin aqueous dissolution or suspension. One convenient alkaline materialwhich can be used is an alkali metal hydroxide, usually sodiumhydroxide. Another convenient material is an alkaline earth metalhydroxide, usually a lime slurry or a limestone slurry.

Absorption of sulfur dioxide in an aqueous alkaline medium tends toproduce the corresponding sulfite. It is preferred, however, that thereaction product be the corresponding sulfate, in view of the greatereconomic attraction of the sulfate salts. For example, where lime orlimestone slurry is used, the by-product is calcium sulfate (gypsum), amulti-use chemical.

Accordingly, in a preferred aspect of the invention, anoxygen-containing gas stream, which usually is air but which may be pureoxygen or oxygen-enriched air, analogously to the case of hydrogensulfide, also is introduced to the aqueous alkaline reaction medium, soas to cause the sulfate salt to be formed. When such oxidation reactionis effected in the presence of a lime or limestone slurry, it isgenerally preferred to add a small amount of an anti-caking agent, toprevent caking of the by-product calcium sulfate on the lime orlimestone particles, decreasing their effectiveness. One suitableanti-caking agent is magnesium sulfate.

The concentration of sulfate salt builds up in the aqueous solutionafter initial start up until it saturates the solution, whereupon thesulfate commences to precipitate from the solution. The crystallinesulfate, usually sodium sulfate or calcium sulfate crystals, may befloated from the solution by the sulfur dioxide depleted gas bubbles, ifdesired, with the aid of flotation-enhancing chemicals, if required.

The oxygen-containing gas stream, when used, may be introduced to theaqueous medium at the same submerged location as the sulfurdioxide-containing gas stream, either in admixture with the sulfurdioxide-containing gas stream or as a separate gas stream.

Alternatively, the oxygen-containing gas stream may be introduced to theaqueous alkaline medium at a different submerged location from thesulfur dioxide-containing gas stream using a second impeller/shroudcombination, as described in more detail in the aforementioned copendingU.S. patent application Ser. No. 709,158.

The process of the invention is capable of rapidly and efficientlyremoving sulfur dioxide from gas streams containing the same. Such gasstreams may contain any concentration of sulfur dioxide and the processis capable of removing such sulfur dioxide in efficiencies exceeding99.99%. Residual sulfur dioxide concentrations below 0.1 ppm by volumecan be achieved.

This sulfur dioxide removal embodiment of the invention can be carriedout under a variety of process conditions, the choice of conditionsdepending, to some extent, on the chemical imparting alkalinity to thereaction medium. For an alkali metal hydroxide, the aqueous alkalinesolution generally has a concentration of from about 50 to about 500g/L. For an alkaline earth metal hydroxide, the aqueous alkalinesolution generally has a concentration of from about 1 to about 20 wt %.The active alkalinating agent may be continuously and intermittentlyreplenished to make up for the conversion to the corresponding sulfiteor sulfate. The reaction temperature may vary widely from about 5° toabout 80° C.

In addition to the removal of gaseous components from a gas stream asparticularly described above, the procedure of the present invention,employing the impeller-shroud combination, and the operating parametersof impeller tip speed velocity and gas velocity index through theshroud, also may be used in other instances where distribution of gasphase in a liquid phase is desired and intimate contact of gas andliquid phases is desired.

For example, the procedure of the invention may be employed in wastewater treatment, where undesired dissolved components in the liquidphase, including both BOD and COD, are removed by oxygenation by oxygencontained in a gas stream and dispersed as fine bubbles in the liquidphase in the manner described above.

Another example involves the blending of wood pulp in a slurry in theliquid phase by dispersing oxygen, ozone or a mixture of such gases, asfine gas bubble in the liquid phase to provide an intimate contact ofgas, liquid and solid phases to achieve oxidation of components of thewood pulp fibers.

Another application of the process of the invention in the pulp andpaper industry is the oxidation of components of white liquor. Whiteliquor is a solution of sodium sulfide and sodium hydroxide used to formwood pulp from wood chips. Oxidation of such material may be achieved bydispersing oxygen or an oxygen-containing gas stream in the white liquorusing the procedures described herein.

The dispersion of gaseous phase in the liquid phase in the presentinvention may be combined with other components to effect the desiredreaction or interaction between gaseous and liquid phase components. Asdescribed above, such additional component may comprise a catalystdissolved in the liquid phase promoting reaction between gaseouscomponents.

Another example is the removal of the volatile organic compounds(VOC's), as well as semi-volatile organic chemicals, from aqueousstreams using an oxygen-containing gas distributed as fine bubbles withthe impeller and shroud combination and the process conditions describedherein. The shroud and/or impeller may be coated with a solid catalystfor catalyzing oxidation of the VOCs to carbon dioxide and otherharmless oxidation products.

In this procedure, a multi-stage unit may be employed consisting ofthree or four independent impeller systems, together with a feed ofoxygen or other oxygen-containing gas, such as air, possibly underpressure. The water to be treated to remove VOCs then may be introducedat one end of the series of contactors and removed, after treatment, atthe other end. In each contacting stage, the VOCs are stripped from theaqueous phase by the fine bubbles of oxygen-containing gas and, at thesame time, converted by oxidation in contact with the catalyst to carbondioxide, water and perhaps chlorine or hydrogen chloride, depending onthe chemical nature of the VOCs treated. At the levels of volatileorganic contamination generally found, these impurities do not createsignificant difficulties for the process.

There are several advantages to this VOCs-treatment process. Theprocedure operates economically under significantly higher oxygenpressure than normally encountered with air, which tends to increasemass transfer and chemical reaction rates significantly, since VOCoxidation is usually a first order reaction in terms of oxygen partialpressure. No bleed of oxygen containing stripped VOCs is required, sincethe main products are carbon dioxide, water and the minor impuritiesmentioned above, which achieve an equilibrium state with the oxygen.

Alternatively, the VOCs may be stripped from the aqueous phase by theair stream distributed in the aqueous phase to form a gas streamcontaining such stripped VOCs, which then may be passed in contact withthe catalyst for oxidation of VOCs to carbon dioxide and similaroxidation products external to the stripping operation.

If desired, liquid circulation within the contactor can be controlled bya series of overflow-underflow weirs, ensuring good gas-liquid contactand a reasonable residence time of liquid within each contact cell. Theliquid flow rate can be modified over a wide range depending on thecontaminant level and activity.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, a novel gas-liquid contact apparatus 10,provided in accordance with one embodiment of the invention, is amodified form of an agitated flotation cell. The design of thegas-liquid contactor 10 is intended to serve the purpose of efficientlycontacting gases and liquids, for example, to effect removal of acomponent of the gas, such as by reaction to produce a flotableinsoluble phase, but also applicable to the chemical conversion ofaqueous phase components by gaseous phase components dispersed in theliquid phase. This design differs from that of an agitated flotationcell whose objective is to separate a slurry or suspension into aconcentrate and a gangue or barren stream.

There are significant differences between a conventional agitatedflotation cell and the modified flotation cell 10 of the presentinvention which arise from the differences in requirements of the twodesigns. In the present invention, the substances which are treated maybe contained in the gas stream or the liquid phase whereas, in anagitated flotation cell, the substances which are treated are the solidphase contained within the slurry and the gas is employed solely tofloat the desired particles out of the slurry without chemical or othermodification to such particles. To the extent the present inventioninvolves treatment of components in the liquid phase, such treatmentgenerally involves chemical interaction of components.

An agitated flotation cell is designed to process a slurry or suspensionto effect physical separation of a solid phase. The capacity of the cellis measured as the volume of treated slurry in a given time and theefficiency is measured as the mass fraction of desired mineralphysically separated relative to that in the entering slurry orsuspension. Normally, a number of stages is required in such mineralseparation operation, including a roughing stage to effect thenon-reactive separation. In contrast, an apparatus which removes acomponent from a gas stream by chemical reaction or physical separation,as in the case of device 10, is engineered to process and treat a flowof gas. Capacity is measured in volume of gas throughput and efficiencyis measured in terms of the relative removal as compared to the desiredremoval. Normally, only one separation step is required.

In addition, an agitated flotation cell is designed to generate amultiplicity of small air bubbles which are distributed uniformlythroughout the slurry by means of a shroud to ensure good contactingbetween gas bubbles and the desired mineral particles within the body ofthe slurry external to the shroud. Normally, no chemical reaction takesplace in the cell but surface-active agents may be added to change theflotability of the concentrate. In contrast, in a chemical reactor, suchas gas-liquid contact device 10, the contacting and reaction chemistryare of paramount importance and directly affect the efficiency of theunit, whether involving reaction of gaseous phase components one withanother or reaction of gaseous phase components with liquid phasecomponents. The conditions which exist in the interior and to theimmediate exterior of the shroud are critical for the purposes of thepresent invention. Effective contacting between gas phase and liquidphase is achieved in the present invention to effect chemical andphysical separation operations by rotation of the impeller at rates wellin excess of those used in an agitated flotation cell and by utilizing ashroud with much smaller openings, leading to a much higher gas velocityindex than used as agitated flotation cell. The reactor 10 as an H₂ Sreactor utilizes a chemical reaction in which hydrogen sulfide isoxidized through the medium of a catalyst by oxygen in the interior ofthe shroud and to the immediate exterior of the shroud. The flotation ofsulfur by hydrogen-sulfide depleted gas bubbles is a very significantadditional benefit in the operation of the reactor but is not a primarydesign criterion.

In a conventional agitated flotation cell, the impeller is smallrelative to the size of the flotation cell, since its purpose is toproduce a myriad of small bubbles to be distributed and dispersedthrough the liquid slurry and not to promote efficient gas-liquidcontacting. The shroud is designed with relatively few large openings todistribute the small bubbles produced by the impeller uniformly in thecell, ensuring good contacting between the bubbles and the desiredcontacting phase to promote the desired phase separation. The bubblesare maintained within a relatively narrow size range to ensure a largesurface area for gas-solid contacting, not gas-liquid contacting asdesired herein, and the bubbles are active throughout the entire volumeof the cell, otherwise the desired solid phase separation would not beachieved. As conventional agitated flotation cells increase in size, theproportion of liquid pumped through the shroud increases and themomentum of the liquid carries the bubbles required for flotation to theouter reaches of the cell.

In contrast, in the gas-liquid contactor herein, the impeller may belarger relative to the size of the reactor and its design may be alteredto increase the efficiency of gas-liquid contacting. Most of thechemical or physical process occurs very close to the impeller, to theinterior of the shroud and to its immediate exterior, so that theeffective or active zone is a much smaller fraction of cell volume thanin the case of flotation where separation in the bulk is required. Theshroud is designed herein with a large number of smaller openings, whichusually have sharp edges (i.e. the surfaces intersect at an acute angle)to promote secondary contacting by which gas shearing further improvesthe efficiency of the gas-liquid contacting reaction.

In the apparatus 10 of the invention, the gas inlets and outlets aremuch larger than in a conventional flotation cell to accommodate anincreased flow of gas. Similarly, liquid inlets and outlets aresufficient for the purposes of filling and draining the vessel, but notfor the continuous flow of slurry as in the case of the agitatedflotation cell.

The reactor 10, constructed in accordance with one embodiment of theinvention and useful in chemical and physical processes for removing acomponent from a gas stream, such as oxidative removal of hydrogensulfide, and other gas-liquid contacting processes, such as describedabove, comprises an enclosed housing 12 having a standpipe 14 extendingfrom exterior to the upper wall 16 of the housing 12 downwardly into thehousing 12. The housing 12 may be of any convenient shape, generallyrectangular. The housing 12 may be designed such as to avoid dead zonesin the liquid phase contained within the housing.

Inlet pipes 18,20 communicate with the standpipe 14 through an inletmanifold at its upper end for feeding a gas stream, in this illustratedembodiment, hydrogen sulfide-containing gas stream and air to reactor10. The inlet pipes 18,20 have inlet openings 22,24 through which thegas flows. The openings are designed to provide a low pressure drop.

Generally, the flow rate of gas streams may range upwardly from aminimum of about 50 cu.ft/min., for example, in excess of about 500cu.ft/min., although much higher or lower flow rates may be employed,depending on the intended application of the process. The pressure dropacross the unit may be quite low and may vary from about -5 to about +10in. H₂ O, preferably from about 0 to less than about 5 in. H₂ O. Forlarger units employing a fan or a blower to assist the gas flow rate tothe impeller, the pressure drop may be greater.

A shaft 26 extends through the standpipe 14 and has an impeller 28mounted at its lower end just below the lower extremity of the standpipe14. A drive motor 30 is mounted to drive the shaft 26. Although there isillustrated in the drawings an apparatus 10 with a single impeller 28,it is possible to provide more than one impeller and hence more than oneoxidative reaction (or other chemical or physical process) location inthe same enclosed tank. The gas flow rate to the reactor referred toabove represents the flow rate per impeller.

The impeller 28 comprises a plurality of radially-extending blades 32.The number of such blades may vary and generally at least four bladesare employed, with the individual blades being equi-angularly spacedapart. The impeller is illustrated with the blades 32 extendingvertically. However, other orientations of the blades 32 are possible.

Generally, the standpipe 14 has a diameter dimension related to that ofthe impeller 28 and the ratio of the diameter of the standpipe 14 tothat of the impeller 28 generally may vary from about 1:1 to about 2:1.However, the ratio may be lower, if the impeller is mounted below thestandpipe. The impeller 28 generally has a height which corresponds toan approximately 1:1 ratio with its diameter, but the ratio generallymay vary from about 0.3:1 to about 3:1. As the gas is drawn down throughthe standpipe 14 by the action of the rotary impeller 28 and the liquidphase is drawn into the impeller, the action of gas and liquid flows androtary motion produce a vortex of liquid phase in the upper region ofthe impeller 28. Alternatively, the gas may be introduced below theimpeller and drawn into the interior of the shroud by the action of theimpeller.

The ratio of the projected cross-sectional area of the shrouded impeller28 to the cross-sectional area of the cell may vary widely, and often isless but may be more than in a conventional agitated flotation cell,since the reaction is confined to a small volume of the reaction mediumand will be determined by the ultimate use to which the apparatus 10 isput. The ratio may be as little as about 1:2. However, where additionalprocessing of product is required to be effected efficiently, such asflotation of sulfur, the ratio generally will be higher.

Another function of the impeller 28 is to distribute the induced gasesas small bubbles within the liquid medium in the interior of the shroud.This result is achieved by rotation of the impeller 28, resulting inshear of liquid and gases to form very fine bubbles dimensioned so thatthe largest are no more than about 1/4 inch in diameter. Generally, thebubbles are dimensioned so that, as they leave the shroud openings, thelarge majority of the bubbles all have a dimension less than about 2 mm,and typically about 0.5 to 0.7 mm diameter. In this way, a gas-liquidmixture of fine gas bubbles in the liquid phase is formed containedwithin the shroud 34.

A critical parameter in determining an adequate shearing to form the gasbubbles is the velocity of the outer tip of the blades 32. A blade tipvelocity of at least about 350 in/sec is required to achieve efficient(i.e., 99.99%+) removal of hydrogen sulfide, preferably about 500 toabout 700 in/sec. This blade tip velocity is much higher than typicallyused in a conventional agitated flotation cell, wherein the maximumvelocity is about 275 in/sec.

The impeller 28 is surrounded by a cylindrical stationary shroud 34having a uniform array of circular openings 36 through the wall thereof.The shroud 34 generally has a diameter slightly greater than thestandpipe 14. Although, in the illustrated embodiment, the shroud 34 isright cylindrical and stationary, it is possible for the shroud 34 topossess other shapes. For example, the shroud 34 may be tapered, withthe impeller 28 optionally also being tapered. In addition, the shroud34 may be rotated, if desired, usually in the opposite direction to theimpeller 28. Further, the shroud 34 is shown as a separate element fromthe standpipe 14. However, the shroud 34 may be provided as an extensionof the standpipe, if desired.

Further, the openings 36 in the shroud are illustrated as beingcircular, since this structure is convenient. However, it is possiblefor the openings to have different geometrical shapes, such as square,rectangular or hexagonal. Further, all the openings 36 need not be ofthe same shape or size.

The shroud 34 serves a multiple function in the device. Thus, the shroud34 prevents gases from by-passing the impeller 28, assists in theformation of the vortex in the liquid necessary for gas induction,assists in achieving shearing as well as providing additional shearingand confines the gas-liquid mixture and hence maintains the turbulenceproduced by the impeller 28. The effect of the impeller-shroudcombination may be enhanced by the employment of a series of elongatebaffles, provided on the internal wall of the shroud 34, preferablyvertically extending from the lower end to the upper end of the openingsin the shroud. The gas-liquid mixture flows through and in contact withthe openings 36 in the shroud which results in further shearing of thefine gas bubbles and further intimate contact of the gaseous and liquidphases.

The shroud 34 is spaced only a short distance from the extremity of theimpeller blades 30, in order to provide and promote the above-notedfunctions. Generally, the ratio of the diameter of the shroud 34 to thatof the impeller 28 generally is about 3:1 to about 1.1:1, preferablyapproximately 1.5:1.

In contrast to the shroud in a conventional agitated flotation cell, theopenings 36 generally are larger in number and smaller in diameter, inorder to provide an increased area for shearing, although an equivalenteffect can be achieved using openings of large aspect ratio, such asslits. When such circular openings are employed, the openings 36generally are uniformly distributed over the wall of the shroud 34 andusually are of equal size. The equivalent diameter of the openings 36often is less than about one inch and generally should be as small aspossible without plugging, preferably about 3/8 to about 5/8 inch indiameter, in order to provide for the required gas flow therethrough.When the openings 36 are of non-circular geometrical shape and of aspectratio which is approximately unity, then the area of each such opening36 generally is, less than the area of a circular opening having anequivalent diameter of about one inch, preferably about 3/8 to about 5/8inch. The openings have sharp corners to promote shearing of the gasbubbles passing through the openings and contacting the edges.

The openings 36 are dimensioned to permit a gas flow rate therethroughcorresponding to less than about 0.02 lb/min/shroud opening, generallydown to about 0.004 lb/min/shroud opening. As noted earlier, the gasflow rate may be higher at or near the upper limit of capacity of theunit. Preferably, the gas flow rate through the shroud openings is about0.005 to about 0.007 lb/min/opening in the shroud. As noted above, ingeneral, the gas velocity index is at least about 18 per second peropening in the shroud, preferably at least about 24 per second peropening, and more preferably at least about 30 per second per opening.

As a typical example, in a conventional agitated flotation cell,forty-eight circular openings 1.25 inches in diameter for acircumferential length of 188 inches may be employed while, in the samesize unit constructed as a reactor in accordance with the presentinvention, 670 circular openings each 3/8-inch in diameter are used fora total circumferential length of 789 inches. In addition, in thepresent invention the gas flow through the openings is typically 0.007lb/min/opening (a gas velocity index of 65 per second per opening) inthe shroud, while in a conventional agitated flotation cell of the sameunit size the same parameter is 0.03 lb/min/opening (a gas velocityindex less than 10 per second per opening) in the shroud. As may be seenfrom this typical comparison, the physical dimensions of the openingsand the gas flow are significantly different in the gas-liquid contactdevice of this invention from those in an agitated flotation cell.

The spacing between openings is largely dictated by considerations ofadequacy of structural strength and the desired liquid and gas flowintroduction. Generally, each circular opening, is spaced from about0.25 to about 0.75 of the diameter of the opening from each other,typically about 0.5, although other arrangements are possible.Generally, the plurality of openings is arranged at a density of lessthan about 2 per square inch in a regular array.

The shroud 34 is illustrated as extending downwardly for the height ofthe impeller 28. It is possible for the shroud 34 to extend below theheight of the impeller 28 or for less than its full height, if desired.

In addition, in the illustrated embodiment, the impeller 28 is located adistance corresponding approximately half the diameter of the impeller28 from the bottom wall of the reactor 10. It is possible for thisdimension to vary from no less than about 0.25:1 to about 1:1 or greaterof the proportion of the diameter dimension of the impeller. Thisspacing of the impeller 28 from the lower wall allows liquid phase to bedrawn into the area between the impeller 28 and the shroud 34 from themass in the reactor. If desired, a draft tube may be provided extendinginto the body of the liquid phase from the lower end of the impeller, toguide liquid into the region of the impeller.

By distributing the gases in the form of tiny bubbles and effectingshearing of the bubbles in contact with the iron chelate solution withinthe shroud 34 and during passage through the openings 36 therein, rapidmass transfer occurs and the hydrogen sulfide is rapidly oxidized tosulfur. The reaction occurs largely in the immediate region of theimpeller 28 and shroud 34 and forms sulfur and hydrogen sulfide-depletedgas bubbles.

The sulfur particles initially remain suspended in the turbulentreaction medium but grow in the body of the reaction medium to a sizewhich enables them to be floated by the hydrogen sulfide-depleted gasbubbles. When the sulfur particles have reached a size in the range ofabout 10 to about 50 microns in diameter, they possess sufficientinertia to penetrate the boundary layer of the gas bubbles to therebyenable them to be floated by the upwardly flowing hydrogensulfide-depleted gas bubbles.

Other odiferous components of the hydrogen sulfide-containing gasstream, such as mercaptans, disulfides and odiferous nitrogenouscompounds, such as putrescenes and cadaversenes, also may be removed byadsorption on the sulfur particles.

At the surface of the aqueous reaction medium, the floated sulfuraccumulates as a froth 38 and the hydrogen sulfide-depleted gas bubblesenter an atmosphere 40 of such gas above the reaction medium 42. Thepresence of the froth 38 tends to inhibit entrainment of an aerosol ofreaction medium in the atmosphere 40.

A hydrogen sulfide-depleted gas flow outlet 44 is provided in the upperclosure 16 to permit the treated gas stream to pass out of the reactorvessel 12.

An adequate freeboard above the liquid level in the reaction vessel isprovided greater than the thickness of the sulfur-laden froth 38, tofurther inhibit aerosol entrainment.

Paddle wheels 46 are provided adjacent the edges of the vessel 12 inoperative relation with the sulfur-laden froth 38, so as to skim thesulfur-laden froth from the surface of the reaction medium 42 intocollecting launders 48 provided at each side of the vessel 12. Theskimmed sulfur is removed periodically or continuously from the launders48 for further processing.

The sulfur is obtained in the form of froth containing about 15 to about50 wt. % sulfur in reaction medium. Since the sulfur is in the form ofparticles of a relatively narrow particle size, the sulfur is readilyseparated from the entrained reaction medium, which is returned to thereactor 10.

The gas-liquid contact apparatus 10 provides a very compact unit whichrapidly and efficiently removes hydrogen sulfide from gas streamscontaining the same. Such gas streams may have a wide range ofconcentrations of hydrogen sulfide. The compact nature of the unit leadsto considerable economies, both in terms of capital cost and operatingcost, when compared to conventional hydrogen sulfide-removal systems.

There has previously been described in U.S. Pat. No. 3,993,563 a gasingestion and mixing device of the general type described herein. Inthat reference, it is indicated that, for the device described therein,if an increase in the rotor speed is made in an attempt to obtaingreater gas-liquid mixing action, then it is necessary to employ abaffle in the standpipe in order to obtain satisfactory gas ingestion.As is apparent from the description herein, such a baffle is notrequired in the present invention.

However, with larger size units designed to handle large volumes of gas,it may be desirable to provide a conical perforated hood structure abovethe impeller-shroud combination to quieten the surface of the liquidmedium in the vessel.

Referring now to FIG. 4, there is illustrated therein an alternativeembodiment of apparatus 100 provided by this invention. Elements of thisapparatus 100 which are in common with those employed in apparatus 10 ofFIGS. 1 to 3 have been designated by the same reference numerals as usedtherein and a description of their construction and operation will notbe repeated.

In apparatus 100, a baffle arrangement 102 surrounds and is spaced fromthe shroud 34. The baffle 102 has a lower opening 104 to permit reactionmedium 42 to be drawn by the action of the impeller 28 into the interiorof the shroud 34. The baffle 102 has a cylindrical wall 104 whichterminates at its upper extremity above the liquid level of the reactionmedium 42 and hence defining an annular flow path for gas, liquid andany product of reaction thereof external of the shroud 34 to the gasspaced 40 for separation of gas and liquid phase thereat.

EXAMPLES Example 1

A pilot plant apparatus was constructed as schematically shown in FIG. 1and was tested for efficiency of removal of hydrogen sulfide from a gasstream containing the same.

The overall liquid capacity of the tank was 135 L. The standpipe had aninside diameter of 71/2 in., and the impeller consisted of six bladesand had a diameter of 51/2 in. and a height of 61/4 in. and waspositioned 21/4 in. from the base of the tank.

The pilot plant apparatus, fitted with a standard froth flotation shroudand impeller combination, was charged with 110 L of an aqueous solutionwhich contained 0.016 mol/L of ethylenediaminetetraacetic acid,iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. ThepH of the aqueous medium was 8.5. The shroud consisted of a stationarycylinder of outside diameter 12 in., height 53/4 in and thickness 3/4in. in which was formed 48 circular openings each 1.25 in. in diameter,for a total circumferential length of 188 inches.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 835 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 733 rpm., corresponding to a blade tip velocity of about 211 in/sec.The gas velocity index through the shroud openings was 11.7 per secondper opening in the shroud. (The gas flow rate was 0.05 lb/min/opening.)Over the one and a half hour test period, 99.5% of the hydrogen sulfidewas removed from the gas stream, leaving a residual amount of H₂ S inthe gas stream of 20 ppm. Sulphur was formed and appeared as a froth onthe surface of the aqueous solution and was skimmed from the surfaceusing the paddle wheels. Simultaneous removal of hydrogen sulfide fromthe gas stream and recovery of the sulfur produced thereby, therefore,was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during the period of the test.

Example 2

The procedure of Example 1 was repeated with an increased impellerrotation rate and higher gas flow rate.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1772 rpm corresponding to a blade tip velocity of about 510 in/sec.The gas velocity index through the shroud openings was 13.7 per secondper opening in the shroud. (The gas flow rate was 0.06 lb/min/opening.)Over the two hour test period 99.7% of the hydrogen sulfide was removedfrom the gas stream, leaving a residual amount of H₂ S of 11 ppm. Sulfurwas formed and appeared as a froth on the surface of the aqueoussolution and was skimmed from the surface. Simultaneous removal ofhydrogen sulfide from the gas stream and recovery of the sulfur producedthereby, therefore, was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during this period of the test.

Example 3

The pilot plant apparatus was modified and fitted with a shroud andimpeller combination as illustrated in FIG. 2, was charged with 110 L ofan aqueous solution which contained 0.016 mol/L ofethylenediaminetetra-acetic acid, iron-ammonium complex and 0.05 mol/Lof sodium hydrogen carbonate. The pH of the aqueous solution was 8.5.The shroud consisted of a stationary cylinder of outside diameter 123/4in., height 81/2 in., and thickness 1/2 in. in which was formed 670openings each of 3/8 in. diameter for a total circumferential length of789 inches. Vertical baffles extending vertically from top to bottom ofthe shroud were provided on the internal wall equally arcuately spaced,ten in number with a 1/4-inch×1/4-inch space cross section. The impellerwas replaced by one having a diameter of 61/2 in. The other dimensionsremained the same.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1754 rpm., corresponding to a blade tip velocity of about 597 in/sec.The gas velocity index through the shroud was 36.3 per second peropening. (The gas flow rate was 0.004 lb/min/opening.) Over the two hourtest period 99.998% of the hydrogen sulfide was removed from the gasstream, leaving a residual amount of H₂ S of less than 0.1 ppm. Sulphurwas formed and appeared as a froth on the surface of the aqueoussolution and was skimmed from the surface. Simultaneous removal ofhydrogen sulfide from the gas stream and recovery of the sulfur producedthereby, therefore, was effected.

During the test period, the pH of the aqueous solution remainedrelatively constant at 8.5. No additional alkali or catalyst was addedduring the period of this test.

As may be seen from a comparison of the results presented in Examples 1,2 and 3, it is possible to remove hydrogen sulfide with greater than 99%efficiency using an agitated flotation cell which is provided with aconventional shroud and impeller construction (Examples 1 and 2), asalready described in Canadian Patent No. 1,212,819. However, byemploying a higher blade tip velocity, as in Example 2, a modestincrease in efficiency can be achieved.

However, as seen in Example 3, with a shroud modified as describedtherein to provide the critical gas flow rate and using the criticalblade tip velocity, efficiency values over 99.99% can be achieved,leaving virtually no residual hydrogen sulfide in the gas stream.

Example 4

The pilot plant apparatus of FIG. 1 was tested for efficiency of removalof sulfur dioxide from a gas stream containing the same. The elements ofthe pilot plant apparatus were dimensioned as described in Example 3.

The pilot plant apparatus was charged with 110 L of an aqueous slurrycontaining 13.2 kg of CaO and 3450 g of MgSO₄.7H₂ O. Air, containingvarying amounts of sulfur dioxide was passed through the apparatus viathe standpipe at varying flow rates at room temperature, while theimpeller in the aqueous slurry rotated at a rate varying from 1760 to1770 rpm, corresponding to a blade tip velocity of 599 to 602 in/sec.The corresponding gas velocity indices through the shroud were from 31.1to 124.5 per second per opening. (The gas flow rates were 0.003 to 0.01lb/min/opening.)

A series of one hour runs was performed and the residual SO₂concentration was measured after 45 minutes. The results obtained areset forth in the following Table I:

                  TABLE I                                                         ______________________________________                                        Gas Flow Rate                                                                              SO.sub.2 Concentration                                           (cfm)        In*.sup.(1)                                                                           (ppmv)    Out*.sup.(2)                                                                        RPM                                      ______________________________________                                        30           1000              <0.4  1760                                     30           5000              <0.4  1760                                     30           7000              <0.4  1760                                     30           10000             0.6   1760                                     60            900              <0.4  1770                                     75           1000              <0.4  1760                                     100          1000              0.8   1763                                     120          1000              5.6   1770                                     ______________________________________                                         Notes:                                                                        .sup.(1) Concentration values vary approximately ± 10%.                    .sup.(2) Concentration values vary approximately ± 0.2 ppm by volume. 

As may be seen from this data, highly efficient (>99.99%) removal ofsulfur dioxide from the gas stream was obtained using a lime slurry,even at high sulfur dioxide concentrations and less efficient removalwere observed only at high gas flow rate.

Example 5

The procedure of Example 4 was repeated using 110 L of an aqueous slurryof 13.2 kg of calcium carbonate and 3450 g of MgSO₄.7H₂ O. In theseexperiments, the impeller was rotated at a speed of 1770 to 1775 rpm,corresponding to a blade tip velocity of 602 to 604 in/sec. Thecorresponding gas velocity index through the shroud were 31.1 to 103.8per second per opening. (The gas flow rates were 0.003 to 0.01lb/min/opening)

The results obtained are set forth in the following Table II:

                  TABLE II                                                        ______________________________________                                        Gas Flow Rate                                                                              SO.sub.2 Concentration                                           (cfm)        In.sup.(1)                                                                            (ppmv)    Out.sup.(2)                                                                         RPM                                      ______________________________________                                        30            900              <0.4  1770                                     30           2000              <0.4  1770                                     30           3000              <0.4  1770                                     30           5000              <0.4  1770                                     30           9000              <0.4  1770                                     30           10000             <0.4  1770                                     45           1000              <0.4  1773                                     60           1000              <0.4  1775                                     75           1050              <0.4  1775                                     100          1000              5.25  1775                                     ______________________________________                                         Notes:                                                                        .sup.(1) Concentration values vary approximately ± 10%.                    .sup.(2) Concentration values vary approximately ± 0.2 ppm by volume       except for last run, approximately ± 1 ppm by volume.                 

As may be seen from this data, highly efficient (>99.99%) removal wasobtained using a limestone slurry, even at high sulfur dioxideconcentrations and less efficient removal were observed only at high gasflow rate.

Example 6

A bench scale reaction was set up corresponding in construction to theapparatus of FIG. 1. 4 L of the catalyst solution described in Example 3was charged to the reactor. An off-gas stream from a feathers cooker ofa meat rendering plant was fed to the reactor along with air and, outerthe test period, the pH of the catalyst solution, the rpm at which theimpeller turned, the pressure difference between the reactor standpipeand the atmosphere and the temperature of the off-gas stream were allmonitored. Gas analysis for hydrogen sulfide and methanethiolconcentrations were effected for reactor feed and exit streams.

The separate runs were effected and the results obtained are summarizedin the following Tables III and IV separately:

                  TABLE III                                                       ______________________________________                                                           ΔP                                                                             T    H.sub.2 S.sub.IN                                                                    H.sub.2 S.sub.OUT                                                                    Q                                 Time pH     rpm    "H.sub.2 O                                                                           °C.                                                                         ppmv  ppmv   L/Min                             ______________________________________                                        11:20                                                                              8.5    2120   -7.6   33   900   --     26                                12:00                                                                              8.9    1810   -6.8   30   150   <0.1   20                                13:00                                                                              8.9    2060   -7.6   30    33   --     25                                14:00                                                                              8.7    2190   -8.2   37   550   <0.1   28                                15:00                                                                              8.8    2020   -9.8   40    85   <0.1   31                                16:00                                                                              8.7    2060   -7.6   37   1400  <0.1   25                                17:00                                                                              8.5    2370   -8.2   50   700   <0.1   30                                ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                           ΔP                                                                             T    H.sub.2 S.sub.IN                                                                    H.sub.2 S.sub.OUT                                                                    Q                                 Time pH     rpm    "H.sub.2 O                                                                           °C.                                                                         ppmv  ppmv   L/Min                             ______________________________________                                        10:10                                                                              9.0    2230   -9.2   36   --    --     32                                11:00                                                                              8.8    2200   -8.5   44   1100  <0.1   30                                12:00                                                                              8.6    2200   -8.4   45   2000  <0.1   29                                13:00                                                                              8.6    2200   -8.7   48   7500  <0.1   30                                14:00                                                                              8.7    2209   -8.5   49    250  <0.1   30                                15:00                                                                              8.4    2270   -7.4   46    200  <0.1   27                                16:00                                                                              8.3    2300   -8.2   48   1400  <0.1   29                                17:00                                                                              8.5    2320   -7.2   40    85   <0.1   25                                ______________________________________                                    

In these Tables, the following abbreviations are used:

pH: of the catalyst solution

rpm: of the reactor impeller

ΔP: pressure difference between the reactor stand-pipe and theatmosphere ("H₂ O)

T: temperature of the slip-stream at the point where it is removed fromthe feather cooker off-gas (°C.)

H₂ S_(IN) : the hydrogen sulphide concentration in the reactor feed gasstream (ppmv)

H₂ S_(OUT) : the hydrogen sulphide concentration in the reactor exit gasstream (ppmv)

Q: the volumetric flow rate of the reactor feed gas stream (L/min)

During the Table III run, the methanethiol concentration was measured at14:00 in the reactor inlet and outlet gas streams at 8 ppmv and <0.1 pmvrespectively. When the reactor was stopped at 17:15, the pH of thecatalyst solution was 8.5 and the pressure inside the off-gas duct was-3.2 "H₂ O.

During the Table IV run, the methanethiol concentration was measured at12:00 in the reactor inlet and outlet gas streams at 5 ppmv and <0.1ppmv respectively. When the reactor was stopped at 17:00, the pressureinside the off-gas pipe was -5.4 "H₂ O.

After approximately 20 minutes of operation from start-up of the TableIII run, sulfur particles clouded the catalyst solution and after aboutone hour a froth layer of elemental sulfur developed on the surface ofthe catalyst solution. During the two runs, no sulfur was removed fromthe reactor during the runs except for sulfur suspended in solution whencatalyst samples were withdrawn from the reactor. The sulfur particlesincreased in size as the tests proceeded as observed by the length oftime required for the sulfur particles to settle in the catalyst sampleremoved from the reactor.

As can be seen from the results set forth in Tables III and IV, avariable feed concentration of hydrogen sulfide was decreased to belowthe limit of detection of the test equipment (0.1 ppmv) as well asdecreasing the methanethiol concentration below the level of detectionof the test equipment (0.1 ppmv).

The highly odiferous off-gas stream contained a variety of nitrogenousand sulfurous organic compounds, in addition to hydrogen sulfide. Thesecompounds which included the methanethiol, were removed from the gasstream by adsorption by the the sulfur froth and the only detectableodor in the exit gas stream from the reactor was that of ammonia. Thislatter observation was confirmed by chromatographic analysis of theinlet and outlet streams, which showed a variety of compounds besideshydrogen sulfide in the gas stream entering the reactor which wereabsent from the exiting gas stream.

Example 7

The apparatus described in Example 3 was operated to test the masstransfer of oxygen from gas phase to liquid phase by employing a readilyoxidizable component dissolved in the liquid phase, namely sodiumsulfite.

In this procedure, the apparatus first was operated for 10 minutes inthe absence of sodium sulfite and the power requirement (P), standpipepressure (ΔP_(s)), gas flow rate (Q_(g)) and quiecsent water level(L_(s)) after shutdown (to determine liquid volume V_(L)) were measured.The sodium sulfite was added and the reactor operated until a non-zerodissolved oxygen concentration was detected, signifying that the sodiumsulfite had been consumed.

From the latter time (t), the K_(la), i.e. the mass transfercoefficient, may be determined from the relationship:

    ΔC.K.sub.la.V.t.=consumed oxygen in Kg

where ΔC is determined from the equilibrium dissolved oxygen level aftercompletion of the run. ΔC=Ce-O where Ce is the equilibrium oxygenconcentration in the liquid phase.

These runs have been performed, as summarized in the following Table V:

                                      TABLE V                                     __________________________________________________________________________    Q.sub.g   ΔP.sub.s                                                                    P   L.sub.g                                                                             Ce  t   K.sub.la                                      (CFM)     ("H.sub.2 O)                                                                      (HP)                                                                              (mm)  (mg/l)                                                                            (secs)                                                                            (hr.sup.-1)                                   __________________________________________________________________________    Run No. 1                                                                           178 0   13.8                                                                              576   8.6 59.2                                                                              1340                                                            (1.07M.sup.3)                                               Run No. 2                                                                           374 4.5 8.0 484   8.6 61.2                                                                              1300                                                            (0.837M.sup.3)                                              Run No. 3                                                                           553 4.5 7.5 463   8.6 53.8                                                                              1480                                                            (0.799M.sup.3)                                              __________________________________________________________________________

As may be seen from these results high levels of mass transfer wereobserved for different flow rates of air to the reactor.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides novelmethod and apparatus for effecting gas-liquid contact for distributionof a gaseous phase in a liquid phase, particularly for the removal ofcomponents from gas streams, such as by chemical reactions or physicalseparation and, if desired, for separating flotable by-products of suchreactions using an agitated flotation cell, modified in certain criticalrespects to function as an efficient gas-liquid contactor. Modificationsare possible within the scope of this invention.

What we claim is:
 1. A method for the distribution of a gaseous phase ina liquid phase, which comprises:providing a rotary impeller comprising aplurality of blades at a submerged location in said liquid phasesurrounded by a shroud through which are formed a plurality of openings,feeding said gaseous phase to said submerged location, rotating saidimpeller about a substantially vertical axis at a speed corresponding toa blade tip velocity of at least about 350 in/sec so as to draw liquidphase into the interior of the shroud and to generate sufficient shearforces between said impeller and said plurality of openings in saidshroud to distribute said gaseous phase as fine bubbles in said liquidphase to the interior of said shroud and to effect intimate contact ofsaid gaseous phase and said liquid phase at said submerged location soas to form a gas-liquid mixture of fine bubbles of said gaseous phase insaid liquid phase contained within said shroud, flowing said gas-liquidmixture of fine bubbles of gaseous phase and liquid phase from interiorof said shroud through and in contact with said openings to external ofsaid shroud at a gas velocity index (GVI) of at least about 18 persecond per opening in said shroud so as to effect further shearing ofthe fine gas bubbles and further intimate contact of said gaseous phaseand said liquid phase, said gas velocity index being the ratio of thelinear velocity (V) of the gaseous phase through each opening and theequivalent diameter (d) of the opening, as determined by the expression:

    GVI=V/d

where d is determined for each opening by the expression:

    d=4A/P

where A is the area of the opening and P is the length of the perimeterof the opening.
 2. The method of claim 1 wherein said blade tip velocityis about 500 to about 700 in/sec.
 3. The method of claim 1 wherein saidgas velocity index is at least 24 per second per opening.
 4. The methodof claim 2 wherein said gas velocity index is from about 30 to about 400per second per opening.
 5. The method of claim 1 wherein said rotationof said impeller is effected at such a speed and said flow through andin contact with said shroud openings is effected at such a gas velocityindex that a substantial majority of said fine bubbles exiting saidshroud openings are dimensioned less than about 1 mm.
 6. The method ofclaim 1 wherein said fine gas bubbles and liquid medium are flowed fromthe interior of the shroud through the openings to an annular flow pathout of fluid flow communication with a body of said liquid phase exceptat the lower and upper extremity thereof.
 7. The method of claim 1wherein said fine gas bubbles and liquid medium are flowed from theinterior of the shroud through the openings directly into a body of saidliquid phase surrounding said shroud.
 8. The method of claim 1 whereinsaid liquid phase contains an-undesired oxidizable dissolved componentand said gaseous phase comprises oxygen, so as to remove said dissolvedcomponent from said liquid phase.
 9. The method of claim 8 wherein saidliquid phase comprises waste water and said undesired dissolvedcomponent comprises BOD and/or COD material.